1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for releasing an uplink radio resource without errors in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
A multiple-input multiple-output (MIMO) technique improves efficiency of data transmission and reception by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas. Ever since the MIMO technique was introduced from the IEEE 802.16a standard, the MIMO technique has been constantly updated up to now.
The MIMO technique can be classified into a spatial multiplexing technique and a spatial diversity technique. According to the spatial multiplexing technique, different pieces of data are simultaneously transmitted and thus data can be transmitted at a high speed without increasing a system bandwidth. According to the spatial diversity technique, the same data is transmitted through multiple Tx antennas to obtain diversity, thereby increasing data reliability.
A mobile station (MS) can transmit a feedback in uplink. The feedback includes channel information required for data transmission. By using the feedback received from the MS, a base station (BS) can schedule a radio resource and can transmit data. A closed-loop mode is a mode in which data is transmitted by compensating for channel information included in the feedback received from the MS. An open-loop mode is a mode in which data is transmitted without compensating for the channel information included in the feedback received from the MS. The feedback may not be transmitted in the open-loop mode. Even if the feedback is transmitted, the BS may not use the channel information included in the feedback. Generally, in the communication system, the open-loop mode can be used in a channel environment where the MS moves with a high speed, and the closed-loop mode can be used in a channel environment where the MS moves with a slow speed. More specifically, the open-loop mode is used when the MS moves with a high speed since a channel variation is significant and thus channel information including the feedback is unreliable. In addition, the closed-loop mode is used when the MS moves with a slow speed since a channel variation is relatively less significant and thus the channel information including the feedback is reliable and is less sensitive to delay. Examples of a feedback message transmitted by the MS include a scheduling request that requests radio resource allocation such as a bandwidth request (BR), an acknowledgment (ACK)/negative-acknowledgment (NACK) signal that is a response for downlink data transmission, a channel quality indicator (CQI) that indicates downlink channel quality, MIMO information, etc.
Meanwhile, since limited radio resources are allocated to the uplink channel, the uplink channel needs to be frequently allocated and released to transmit the various types of uplink control signals and feedback messages described above. For this, the BS transmits radio resource allocation information to the MS to allocate and release the radio resources. However, if an error occurs in the release of the uplink channel, a new uplink channel to be allocated by the BS may collide with the existing uplink channel which is not released yet. In particular, an uplink control signal transmitted through an uplink control channel has an important role for the operation of the wireless communication system, and thus collision of the uplink control channel may result in deterioration of reliability of the wireless communication system.
Accordingly, there is a need for a method of releasing an uplink radio resource without errors.